1. Field of the Invention
This invention relates to a process for producing a live, attenuated (and cold-adapted) temperature-sensitive vaccine. An aspect of this invention relates to a vaccine for avian infectious bronchitis. Still another aspect of this invention relates to a method for immunizing poultry against avian infectious bronchitis viruses (AIBV). Still another aspect of this invention relates to the live, cold-adapted temperature-sensitive vaccine which is obtained by serial passage of a strain of virus through culture media at suboptimal replication temperatures. A still further aspect of this invention relates to a method for reducing the pathogenicity of an avian infectious bronchitis virus without destroying its immunologic properties. An aspect of this invention specifically relates to Arkansas-type strains of avian infectious bronchitis viruses, whereby these viruses are rendered substantially non-pathogenic without destroying their immunologic properties.
2. Description of the Prior Art
Poultry production accounts for a substantial amount of the total farm income in many areas of the world and can best be described as a large-scale "agribusiness". Like other agribusinesses, its large income-producing ability depends to a great extent upon efficient management. Efficient management has, however, a major disadvantage in that disease risks can be significantly increased and diseases can easily spread through the dense population of birds in a modern poultry farm. Respiratory disease is a particularly severe problem in this particular field of agribusiness and is known to be the cause of substantial business losses. Accordingly, a great deal of scientific expertise and technical skill has been brought to bear on the problem of respiratory disease in poultry, resulting in a variety of strategies for dealing with this problem.
Perhaps the most common strategy involves controlling respiratory disease through vaccination. Currently, there are vaccines available for the viruses of three respiratory diseases, infectious laryngotracheitis, Newcastle disease (ND) and infectious bronchitis (IB). Of these three aspects of avian immunology, AIBV vaccine technology is of particularly great importance. The presently preferred large-scale agribusiness procedure for vaccinating poultry against IB involves the use of live vaccines which are most commonly administered through drinking water, by the eyedrop technique, or by the increasingly popular spray method. As of the present time, the live vaccine appears to provide a far better immunologic response and is the vaccine of choice for broiler chickens and pullets; killed viruses are typically used only as boosters in conjunction with live vaccines and, because of economic constraints; are administered to breeder or layer chickens (e.g. by injection). However, as with any live vaccine, the risk of a pathogenic response may be substantial.
Pathogenicity can be particularly serious in the case of spray vaccination, wherein the poultry are exposed to an aerosol comprising droplets of the vaccine in a suitable liquid carrier. This aerosol technique is used mainly because of its greater savings in labor costs. But despite modification of the live viruses (e.g. attenuation), severe respiratory vaccine reactions in young poultry have been observed in commercial practice.
The success and safety of applying live vaccines by spray depends on several factors, one of which is droplet (particle) size. The smaller the droplet, the greater the potential for severe vaccine reaction. Small droplets may be inhaled directly into lower respiratory tract tissues (lung and airsacs) and may cause disease and mortality particularly when the infection is exacerbated by secondary bacterial agents. Droplet size, however, is difficult or perhaps impossible to control. Realistically, any spray machine is likely to emit droplets of varying size, and environmental factors such as temperature and relative humidity affect the droplet size of a spray.
One approach to the problem of pathogenicity risks in AIBV immunizations has been the development of novel infectious virus strains which are intended to be less virulent but still effective in providing immunity. The following references are believed to be representative of the patent literature relating to modern AIBV technology: U.S. Pat. No. 4,500,638 (Apontoweil et al., February 19, 1985), Re 31,830 (Apontoweil et al., February 12, 1985), 4,481,188 (Apontoweil et al., November 6, 1984) and 4,357,320 (Apontoweil et al., November 2, 1982). Despite recent advances in this field, however, there is still a need for an effective but relatively safer approach to AIBV immunization.
Looking at the entire field of vaccine technology, other approaches to the intertwined goals of safety and effectiveness become apparent, including the technique of attenuation which imparts "temperature sensitivity"to a virus.
Several factors must be considered in attempting to provide a temperature-sensitive (ts) virus. One usual route to temperature sensitivity involves a procedure known as "cold adaptation" (ca) whereby the virus is caused to replicate at a temperature below the optimal temperature of replication, e.g. below 37.degree. C. Unfortunately, not all viruses will reproduce or replicate at suboptimal temperatures. Moreover, even if the virus does replicate at suboptimal temperatures, there is no guarantee that the ca mutant will be sufficiently slow to replicate at internal body temperatures (e.g. about 37.degree. C. in the case of mammals or about 40.degree.-41.5.degree. C. in the case of birds). Finally, even if the ca virus has ts properties, there is no guarantee that the virus will be non-pathogenic at internal body temperatures.
Several workers skilled in the art of developing ts viruses have encountered exactly this problem. Many of the cold-adapted, temperature-sensitive viruses are not entirely desirable as vaccines, because they retain too much pathogenicity at internal body temperatures. Still another problem is that cold adaptation, even at temperatures of 35.degree. C. and less may fail to result in a ts virus. On the other hand, it may be very difficult to obtain any replication at relatively low temperatures, and some viruses cannot replicate at all at temperatures below about 25.degree. or 30.degree. C.
Thus, though ts vaccines for Newcastle disease (ND) have been reported, as in U.S. Pat. No. 4,235,876 to Gits et al. (November 25, 1980), the use of these vaccines has sometimes resulted in undesirable levels of pathogenic response.
According to the patent and scientific literature, there has been some success in developing ts influenza viruses which are avirulent. See Maassab et al., Proc. Soc. Exptl. Bio. Med. 139:768-773 (1972), Maassab, J. Immunology, 102:728-732 (1969), and references cited in these articles. There is also a body of patent and scientific literature regarding ca live Sendai viruses and various bovine viruses. See, for example, U.S. Pat. No. 3,907,986 (Zygraich et al., Sept. 23, 1975), U.S. Pat. No. 3,927,208 (Zygraich et al., Dec. 16, 1975), and U.S. Pat. No. 4,554,158 (Russell, Nov. 19, 1985).